A variety of exhaust manifolds have been known, each of which is connected to a plurality of exhaust ports of an internal combustion engine to guide exhaust gases from the respective exhaust ports to exhaust pipes on a downstream side.
For example, Patent Document 1 below discloses a proposed exhaust manifold in order to achieve both of early activation of a catalyst and improvement in output performance of an internal combustion engine. In the exhaust manifold, three sheet metal members, that is, a front half body, a partition body and a back half body, are superposed on each other. A second exhaust pipe and a third exhaust pipe communicating with a second exhaust port and a third exhaust port are formed between the front half body and the partition body. A first exhaust pipe and a fourth exhaust pipe communicating with a first exhaust port and a fourth exhaust port are formed between the partition body and the back half body.
Patent Document 2 below discloses another proposed exhaust manifold. In the exhaust manifold, an upper shell member and a lower shell member made by press molding are superposed on each other to thereby form first to fourth branch pipe parts and a collecting pipe part, and a partition plate for separating exhaust gases flowing from the first and second branch pipe parts into the collecting pipe part and exhaust gases flowing from the third and fourth pipe parts into the collecting pipe part is attached to the lower shell member. The partition plate merges a pair of the second and third branch pipe parts connected to a second exhaust port and a third exhaust port to thereby form an exhaust gas passage opening in the collecting pipe part.